Abstract: Subcooled boiling process in natural circulation is the basis of safety of third generation nuclear power plant. Moving particle semi-implicit method was employed to simulate this process. In order to improve the surface detection method of condensate droplet, a new global scanning free-surface detection method was proposed. This new free-surface detection method and other typical free-surface detection methods were applied to the simulation of condensate drop impact on wet surface. The results demonstrate that the free-surface detection method based on particle number density, the free-surface detection method based on normal vector and the free-surface detection method based on arc method lead to mislabeled boundary particles due to their defects. The new free-surface detection method overcomes the above defects and detects the free-surface particles accurately. At the same time, the pressure distribution calculated by this new free-surface particles detection method is more close to the physical result. This new free-surface particles detection method has important implications for interfacial heat and mass transfer calculation of condensate droplet.